1. Field of the Invention
The present invention relates to a nonreciprocal circuit element, particularly to a nonreciprocal circuit element capable of matching the input and output characteristic impedances.
2. Description of the Related Art
A lumped-constant nonreciprocal circuit element (isolator) is a high-frequency component for allowing a signal to pass in the transmission direction without loss while blocking a signal traveling in the reverse direction. It is typically used in a transmission circuit of a mobile communication apparatus such as a mobile phone. A known example of such an isolator is described in Japanese Unexamined Patent Application Publication No. 2000-151217.
The isolator described in the Japanese Unexamined Patent Application Publication No. 2000-151217 includes three pairs of central conductors, the three pairs crossing one another at an angle of about 120° relative to one another and being insulated from one another. In this isolator, the two conductors of each pair are not parallel to each other. With this structure, the isolator exhibits wideband electrical characteristics and isolation characteristics in a desired frequency band.
In general, in order to reduce the insertion loss of an isolator, the characteristic impedances of at least two central conductors connected to the input and output terminals of the isolator are preferably matched.
In the isolator described in the Japanese Unexamined Patent Application Publication No. 2000-151217, however, one of the two central conductors connected to the input and output terminals is disposed off the ferrite at their intersection. This means that one of the two central conductors is farther away from the shield plate (common electrode) than the other, the shield plate being disposed on a surface of the ferrite remote from the surface where the central conductors are disposed. Due to this difference between the two central conductors in distance to the ferrite, the characteristic impedances of the central conductors become mismatched, thus the insertion loss increases, and accordingly the transmission efficiency of a signal decreases.
One possible approach for matching the characteristic impedances of two central conductors is to make the width of one central conductor shorter than that of the other. Unfortunately, reducing the width of a central conductor makes the conductor mechanically weak. This is disadvantageous in the production of central conductors.